1. Field of the Invention
The present invention relates to an optical prism (a so-called polarizing beam splitter) as an optical member used for a projection optical system for separating polarization by separating an input beam into two linearly-polarized beams perpendicular to each other, transmitting and outputting one polarized beam, and reflecting the other polarized beam, a projection optical device (projection optical system) employing the polarizing beam splitter, and a projection display such as a liquid projector having the projection optical device as a main part or a rear projection display displaying a projected image on a rear projection screen.
2. Description of the Related Art
Projection displays emitting a light beam from a light source to a small-sized image forming unit forming an optical image corresponding to an image signal and enlarging and projecting the optical image onto a screen by the use of a projection lens so as to obtain a large-sized image are known. An active-matrix liquid crystal panel modulating a light beam using polarization is widely and practically used in the image forming unit. A transmissive or reflective liquid crystal panels can be used as the liquid crystal panel, but the reflective liquid crystal panel enhancing a pixel aperture ratio even with a small size of the liquid crystal panel has attracted attention with the requirement for an increase in brightness, a decrease in size, and an increase in precision of the projection display. In the projection display employing the reflective liquid crystal panel, since an input portion and an output portion of the liquid crystal is the same, it is necessary to separate polarization by the use of a polarizing beam splitter.
FIGS. 29A and 29B are diagrams illustrating a known polarizing beam splitter and a basic optical system of a reflective liquid crystal projector having the polarizing beam splitter and a liquid crystal panel. Here, FIG. 29A is a diagram illustrating a polarization separating function of the polarizing beam splitter and FIG. 29B is a diagram illustrating the basic optical system of the reflective liquid crystal projector.
As shown in FIG. 29A, the known polarizing beam splitter 942 has a structure in which an optical member having a multi-layered film constituting a polarization separating surface 943a is disposed between slopes of two triangular glass prisms (rectangular prisms) 942a and 942b and they are bonded to each other. The polarizing beam splitter 942 has a function of allowing the polarization separating surface 943a to transmit a P polarization component of P and S polarization components input to the polarizing beam splitter 942 and allowing the polarization separating surface 943a to reflect the S polarization component.
As shown in FIG. 29B, a projection optical device 901 as a basic optical system of a projection display having the polarizing beam splitter 942 includes a light emitting unit 912, an image forming unit 914, and a projection lens 917 having an input lens 917a and a transmission lens 917b. 
The light emitting unit 912 emits a light beam to the image forming unit 914 and the image forming unit 914 modulates the light beam on the basis of image information and inputs the modulated light beam as an image-projecting light beam to the input lens 917a of the projection lens 917. The image-projecting light beam input to the input lens 917a is projected onto a screen not shown by the transmission lens 917b of the projection lens 917, whereby an image is projected onto the screen.
The light emitting unit 912 specifically includes a light source (discharge lamp) 922 emitting a predetermined color beam (for example, white beam) and a light-emitting optical system 923. The light-emitting optical system 923 includes a reflecting mirror (parabolic mirror) 924 condensing the beam from the light source 922 and a lens group 925 disposed in front of the light source 922 in a straight line with respect to an optical axis. Only one convex lens is shown as the lens group 925 in the drawings, but for example, a UV cut filter or a condenser lens having a half wavelength plate may be provided.
The image forming unit 914 includes a polarizing beam splitter 942 and a reflective liquid crystal display (hereinafter, also referred to as a reflective liquid crystal panel) 970 generating a light beam corresponding to image information. As described above, the polarizing beam splitter 942 has a function of reflecting or transmitting an input beam depending on the polarization directions of the beam.
For example, the light beam (emitted light) emitted from the light source 922 is condensed by the reflecting mirror 924, is converted into a substantially parallel light beam, and is input to the lens group 925. The light beam having passed through the lens group 925 is condensed and applied to the reflective liquid crystal panel 970 through the polarizing beam splitter 942 having a polarization separating function. The polarizing beam splitter 942 disposed before the reflective liquid crystal panel 970 reflects the S polarization component and transmits the P polarization component, as shown in FIG. 29A. Accordingly, in this configuration, the P polarization component is input to the reflective liquid crystal panel 970.
In the reflective liquid crystal panel 970, the birefringence of liquid crystal varies depending on voltage information of an input image signal SV. That is, the reflective liquid crystal panel 970 applies an electric field to the liquid crystal in accordance with the input image signal SV. The alignment of liquid crystal molecules is changed by the applied electric field. Since the alignment of the liquid crystal molecules has an optical rotatory power, the input beam (P polarization component) polarization-rotates and is then output. In this configuration, while the input beam to the reflective liquid crystal panel 970 passes through the liquid crystal, is reflected by a reflecting film, and passes through the liquid crystal again, the polarization of the light beam is changed from the P polarization component to the S polarization component due to the birefringence and the resultant beam is then output.
The light beam (hereinafter, also referred to as a panel-output beam) output from the reflective liquid crystal panel 970 and spatially modulated on the basis of the image information SV becomes an optical image corresponding to the image signal SV and is input again to the polarizing beam splitter 942. Only the S polarization component (with respect to the polarization separating surface 942c of the polarizing beam splitter 942) of which the vibration direction of polarization has been rotated by the reflective liquid crystal panel 970 is reflected by the polarization separating surface 942c of the polarizing beam splitter 942 and travels to the projection lens 917.
Thereafter, an image is enlarged and projected onto a screen (not shown) by the projection lens 917. That is, an optical image formed by the variation in polarization of a light beam in the reflective liquid crystal panel 970 is enlarged and projected to the screen (not shown), thereby forming a projected image.
The P polarization component of which the polarization is not changed by the reflective liquid crystal panel 970 passes through the polarizing beam splitter 942 and returns to the light emitting unit 912.
As described above, by using the reflective liquid crystal panel 970, a large-sized image with high precision can be displayed by a relatively-small projection optical device 903. However, there is still a need for a small-sized projection display having high brightness and high contrast.
Here, the polarizing beam splitter 942 is formed by bonding the triangular glass prisms 942a and 942b to each other and a multi-layered optical film is deposited and stacked on the bonding surface to form the polarization separating surface 942c, thereby separating the polarization. For example, the polarization separating surface 942c is formed of a polarization separating film using a Brewster angle.
The polarizing beam splitter 942 employing the glass prisms 942a and 942b has great dependency of the P polarization spectrum transmittance and the S polarization spectrum reflectance on an incidence angle, does not provide a good polarization separating characteristic, and does not provide a projected image with a high contrast ratio. Accordingly, in order to obtain a projected image with a high contrast ratio, the incidence angle to the polarizing beam splitter 942 is, for example, in the range of 45±8 degree (air-converted angle) and thus a relatively-dark projection lens 917 having an F number of about F3.5 or more is necessary. That is, in order to improve the polarization separating characteristic (an extinction ratio of transmission or reflection of the P polarized beam and the S polarized beam), it is necessary to input a light beam having a large F number, that is, a light beam close to a parallel beam.
However, in the above-mentioned configuration, it is difficult to increase the brightness due to the restriction of the incident angle to the polarizing beam splitter 942, even using the reflective liquid crystal panel 970 having a small panel size but a high pixel aperture ratio.
Therefore, various suggestions for improving the polarization separating characteristic have been made to solve such a problem. For example, it can be considered that a Vikuiti (registered trademark and/or trademark) DBEF-D film is used in the polarization separating element. There has been suggested a structure in which a wire-grid polarization separating element separating the polarization by the use of a metallic grid formed in minute grid shapes out of metal is inserted between the triangular glass prisms (for example, see JP-A-2003-131212 and JP-A-2006-3384).
By using the wire-grid polarization separating element, a good polarization separating characteristic with respect to a variation in incidence angle of a light beam can be obtained (for example, to cope with 45±15 degree) and the F number of the light-emitting optical unit or the projection lens can be reduced to F2.0 without causing an optical loss or a decrease in contrast of the polarization separating prism, thereby constructing a projection display with high brightness, high contrast, and high precision.